SYNOPSIS

DESCRIPTION

The system provides each process with three interval timers, each decrementing in a
distinct time domain. When any timer expires, a signal is sent to the process, and the
timer (potentially) restarts.
ITIMER_REAL decrements in real time, and delivers SIGALRM upon expiration.
ITIMER_VIRTUAL decrements only when the process is executing, and delivers SIGVTALRM upon
expiration.
ITIMER_PROF decrements both when the process executes and when the system is executing
on behalf of the process. Coupled with ITIMER_VIRTUAL, this timer is
usually used to profile the time spent by the application in user and
kernel space. SIGPROF is delivered upon expiration.
Timer values are defined by the following structures:
struct itimerval {
struct timeval it_interval; /* next value */
struct timeval it_value; /* current value */
};
struct timeval {
time_t tv_sec; /* seconds */
suseconds_t tv_usec; /* microseconds */
};
The function getitimer() fills the structure pointed to by curr_value with the current
setting for the timer specified by which (one of ITIMER_REAL, ITIMER_VIRTUAL, or
ITIMER_PROF). The element it_value is set to the amount of time remaining on the timer,
or zero if the timer is disabled. Similarly, it_interval is set to the reset value.
The function setitimer() sets the specified timer to the value in new_value. If old_value
is non-NULL, the old value of the timer is stored there.
Timers decrement from it_value to zero, generate a signal, and reset to it_interval. A
timer which is set to zero (it_value is zero or the timer expires and it_interval is zero)
stops.
Both tv_sec and tv_usec are significant in determining the duration of a timer.
Timers will never expire before the requested time, but may expire some (short) time
afterward, which depends on the system timer resolution and on the system load; see
time(7). (But see BUGS below.) Upon expiration, a signal will be generated and the timer
reset. If the timer expires while the process is active (always true for ITIMER_VIRTUAL)
the signal will be delivered immediately when generated. Otherwise the delivery will be
offset by a small time dependent on the system loading.

RETURNVALUE

On success, zero is returned. On error, -1 is returned, and errno is set appropriately.

ERRORS

EFAULTnew_value, old_value, or curr_value is not valid a pointer.
EINVALwhich is not one of ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF; or (since Linux
2.6.22) one of the tv_usec fields in the structure pointed to by new_value contains
a value outside the range 0 to 999999.

NOTES

A child created via fork(2) does not inherit its parent's interval timers. Interval
timers are preserved across an execve(2).
POSIX.1 leaves the interaction between setitimer() and the three interfaces alarm(2),
sleep(3), and usleep(3) unspecified.
The standards are silent on the meaning of the call:
setitimer(which, NULL, &old_value);
Many systems (Solaris, the BSDs, and perhaps others) treat this as equivalent to:
getitimer(which, &old_value);
In Linux, this is treated as being equivalent to a call in which the new_value fields are
zero; that is, the timer is disabled. Don'tusethisLinuxmisfeature: it is nonportable
and unnecessary.

BUGS

The generation and delivery of a signal are distinct, and only one instance of each of the
signals listed above may be pending for a process. Under very heavy loading, an
ITIMER_REAL timer may expire before the signal from a previous expiration has been
delivered. The second signal in such an event will be lost.
On Linux kernels before 2.6.16, timer values are represented in jiffies. If a request is
made set a timer with a value whose jiffies representation exceeds MAX_SEC_IN_JIFFIES
(defined in include/linux/jiffies.h), then the timer is silently truncated to this ceiling
value. On Linux/i386 (where, since Linux 2.6.13, the default jiffy is 0.004 seconds),
this means that the ceiling value for a timer is approximately 99.42 days. Since Linux
2.6.16, the kernel uses a different internal representation for times, and this ceiling is
removed.
On certain systems (including i386), Linux kernels before version 2.6.12 have a bug which
will produce premature timer expirations of up to one jiffy under some circumstances.
This bug is fixed in kernel 2.6.12.
POSIX.1-2001 says that setitimer() should fail if a tv_usec value is specified that is
outside of the range 0 to 999999. However, in kernels up to and including 2.6.21, Linux
does not give an error, but instead silently adjusts the corresponding seconds value for
the timer. From kernel 2.6.22 onward, this nonconformance has been repaired: an improper
tv_usec value results in an EINVAL error.